1. Field of the Invention
The present invention relates to bandgap voltage reference circuits, and, more particularly, to temperature-independent bandgap voltage reference circuits having low supply voltage.
2. Description of the Related Art
Applications for portable, battery-operated equipment or systems employing complex, high-performance electronic circuitry have increased recently with the widespread use of cellular telephones, laptop computers, and other systems. One of the essential building blocks for these applications is an integrated circuit (IC) having a low-voltage reference, which may be a bandgap voltage and current reference, to support most analog functions. In such systems, it is desirable for this low-voltage reference to operate at a relatively low power supply voltage, such as on the order of 1.2 to 3.0 volts. Also, it is desirable that the low-voltage reference be stable and substantially immune to temperature variations, power supply variations, and noise.
Typically, a circuit known as a bandgap voltage reference generator is employed to provide the desired stable reference, or bandgap voltage reference. One such bandgap voltage reference generator is described in U.S. Pat. No. 5,512,817, entitled "Bandgap Voltage Reference Generator", by Nagaraj, issued Apr. 30, 1996. Such a generator is particularly useful for a variety of applications; however, bandgap voltage reference circuits as described in the aforementioned patent typically utilize a power supply on the order of about 4 volts to produce a bandgap voltage reference of about 1.25 volts. It may be desirable, in some circumstances, instead to have a current source that produces a current substantially proportional to absolute temperature (PTAT). Such a current source may be employed to provide a bandgap voltage reference, while also providing greater flexibility with respect to alternate applications. A PTAT current source that is capable of providing a current substantially proportional to absolute temperature and operating satisfactorily with a relatively low supply voltage, such as below 4 volts, is described in U.S. Pat. No. 5,646,518 by Lakshmikumar et al. issued Jul. 8, 1997.
Existing 0.35 .mu.m, 3.0-volt bandgap voltage reference circuits have a worst-case simulated temperature variation of about 4% from -40.degree. C. to +125.degree. C. after trimming of the IC. While this worst-case variation may be adequate for most wireless applications, it does not leave adequate margin of operation in some cases. Furthermore, as supply voltages drop below 2.4 volts, the typical 1.24-volt bandgap output voltage is too high for most common-mode voltage applications and must be re-buffered to a lower voltage (typically about Vdd/2), even if no DC load is driven.